Feedback Loops - Is It Game Over For Earth?

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Feedback Loops - Is It Game Over For Earth?

Post by  on Tue Dec 11, 2012 12:40 am

Feedback Loops
  The Potential to Amplify Global Warming Beyond Current Predictions
Date: 22 Nov This article is largely based on "Terrestrial Ecosystem Feedbacks to
  Global Climate Change" by Daniel A. Lashof and Benjamin J. DeAngelo of
  the Natural Resources Defense Council and John Harte and Scott R.
  Saleska of the University of California at Berkeley, submitted for
  publication to the Annual Review of Energy and the Environment, 1997
  edition, volume 22.


“In the next 30 years, we could see an increase in heat
waves like the one now occurring in the eastern United States or the
kind that swept across Europe in 2003 that caused tens of thousands of
fatalities. Those kinds of severe heat events also put enormous stress
on major crops like corn, soybean, cotton and wine grapes, causing a
significant reduction in yields.

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What are feedbacks and why are they important? Current predictions of
  global warming and other climatic changes due to human activities are
  primarily based on sophisticated climate models, the present
  generation of which do not account for important interactions between
  the climate and marine and terrestrial ecosystems. But scientists have
  recognized that changes in these interactions have the potential to
  produce so-called "feedbacks" to the climate system, which could
  either amplify or dampen expected rates of global warming and climatic
  change.

How do feedbacks work? The simple answer is in much the same manner
  that a microphone, amplifier and speaker interact to pick up an
  initial sound, amplify it, project it, and then cycle it though this
  loop again and again until the initial sound becomes an unbearably
  loud tone. This is a classic example of a positive feedback, so-called
  due to this self-reinforcing effect. Both positive and negative (i.e.,
  dampening) feedbacks are possible in the global climate system. Rising
  atmospheric concentrations of greenhouse gases, such as carbon dioxide
  (CO2) and methane (CH4), cause the initial climatic disturbance in the
  form of global warming. Both the increase in greenhouse gases and
  associated warming can affect the structure and function of
  ecosystems, which in turn may accelerate or diminish the warming. The
  end result is global warming and climatic change of a different
  magnitude compared to the initial disturbance.

What does historical evidence say about feedbacks? Historical evidence
  suggests that feedback mechanisms may have played a significant role
  in past climatic fluctuations, such as the apparent cyclical nature of
  the Earth's ice ages. Data spanning the past 220,000 years, for
  example, indicate that variations in temperature and CO2
  concentrations were strongly correlated, with CO2 changes lagging
  behind temperature changes. Thus, it is a fair assumption that past
  warming resulted in the release of greater amounts of CO2, which in
  turn could have led to further warming -- a positive feedback.

Model simulations of past climatic changes reveal that other types of
  positive feedbacks may have been at work as well. Such studies suggest
  that long-term changes in the Earth's orbit -- to which much of the
  historical climatic changes are attributed -- were insufficient to
  account for the full magnitude of past temperature fluctuations.
  Another key factor, then, seems to have been the poleward shift of
  forests into high-latitude grasslands induced by warming; this would
  have reduced the reflectivity (i.e., albedo) of polar regions,
  increased absorption of sunlight, and thus led to greater warming.

Which feedbacks may be most important in the coming "greenhouse"
  century and beyond that are not accounted for in current models?
 
In similar manner to past climatic and large-scale ecosystem
      changes, human-induced global warming is expected to cause a
      poleward shift of forest zones, and thus decrease the reflectivity
      of the Earth's surface, increase absorption of sunlight, and
      enhance rates of warming -- a positive feedback.

Warming in these high-latitude regions may also result in
      increased rates of methane production from moist bogs or
      peatlands. Methane is the second most important greenhouse gas.
      However, likely changes in soil moisture from global climatic
      change will also affect rates of methane emissions, but in less
      certain ways. Future changes in topography in these regions may
      also increase rates of methane release -- a positive feedback.

Warming and associated decreases in soil moisture may bring about
      an increased frequency of natural fires. The burning vegetation
      would pump even more CO2 into the atmosphere -- a positive
      feedback.

Elevated concentrations of CO2 have been shown to cause stunted
      plant transpiration, the process by which plants release water to
      the atmosphere. Transpiration normally acts to cool the surface;
      thus, the result could be even higher regional temperatures at the
      surface -- a positive feedback (although the global implications
      of this are not entirely clear).

In soils, the CO2 "enrichment" could lead to changing ratios among
      important plant nutrients and in the process lead to decreased
      nitrogen availability. In this case, any stimulatory effect that
      increased CO2 may have on plant growth could be constrained -- a
      positive feedback.

Global climatic change is expected to aggravate rates of land
      degradation and desertification, which in turn would result in the
      emission of more disturbed, windblown dust. These particles act to
      cool the surface, on a regional scale, by increasing atmospheric
      reflectivity (when the underlying surface is relatively dark) -- a
      negative feedback.
 
As warming penetrates the ocean sediment layers it could result in
      the release of large amounts of methane, billions of tons of which
      are locked away in an icy mixture, called gas hydrate, that is
      only stable under specific conditions of high pressure and low
      temperatures -- a positive feedback.

Oceanic temperature increases as a result of global warming could
      lead to decreased solubility of CO2, and thus turn some regional
      oceanic CO2 "sinks" into sources -- a positive feedback.

What conclusions should we draw about potential feedbacks? The
  possible feedbacks mentioned here are some of the most important not
  adequately incorporated into current climate models, and it appears
  the net effect of these unaccounted-for processes will be to amplify
  global warming, perhaps substantially, compared with current
  predictions. Based largely on historical evidence, for instance, it
  appears the reflectivity changes associated with poleward shifts of
  forest zones could greatly amplify a warming trend initially induced
  by human activities. The feedbacks involved in the release of methane
  also have a large potential to accelerate global warming.

Even some of the feedbacks which are included in Intergovernmental
  Panel on Climate Change (IPCC) projections may not be adequately
  represented. For example, the negative feedback involving the
  "fertilization effect" of CO2 on plants -- whereby plant growth is
  stimulated and thus more CO2 taken up -- may be largely overestimated
  due to a range of unaccounted-for ecological interactions and
  responses to climatic change that are likely to curtail or even
  reverse the fertilization effect.

In short, there remain many factors which could potentially drive
  rates of global warming and climatic change beyond the current
  projection
s made by the IPCC
. The scientific community is working to
  improve our understanding and representation of such important
  processes. In the meantime, the policymakers will have to take the
  potential impact of these feedbacks into account when formulating
  strategies that seek to minimize the risks of global warming.


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